Because prostate cancer (PCa) requires androgen for development, castration is the primary treatment for patients with late stage PCa. Because androgen receptor (AR) signaling continues to be active in castration-resistant prostate cancer (CRPC), androgen signaling inhibitors (ASI) are becoming the first line treatment for CRPC. However, two FDA-approved ASIs, abiraterone and enzalutamide, only improve overall patient survival for several months. Thus, it is urgent to identify new targets and develop new approaches to treat ASI-resistant CRPC. Metformin, an antidiabetic drug widely used for over 40 years, is now believed to have anti-neoplastic activity in different types of cancers. Recent clinical data suggests that metformin use may reduce development of CRPC and PCa mortality. However, the detailed molecular mechanisms underlying this very important observation is not completely understood. The long-term goals of this study are to identify druggable signaling pathways that offer more effective treatment options for patients with ASI-resistant CRPC. The objective is to define the role of polo-like kinase 1 (Plk1) in regulating the mTOR (mammalian target of rapamycin) pathway and AR signaling, and to exploit this unique pathway as a novel therapeutic target for ASI- resistant CRPC patients. The central hypothesis is that a combination of Plk1 inhibition with metformin is a new approach to treat ASI-resistant CRPC. This hypothesis will be tested by pursuing two Specific Aims - (1) to dissect how Plk1 phosphorylation of TSC1 (tuberous sclerosis complex 1), a negative regulator of the mTOR pathway, leads to activation of the mTOR pathway in vivo; (2) to determine whether a combination of Plk1 inhibition with metformin is a novel approach to treat ASI-resistant CRPC. These two complementary aims will be accomplished using biochemical analyses of signaling intermediates and employing genetic strategies with inducible PCa mouse models, culture systems and PCa xenograft methodologies. The rationale for the research is that it will be the first to probe the importance f Plk1 to the mTOR/AR pathways and to examine whether a combination of Plk1 inhibition with metformin is a novel approach to treat ASI-resistant CRPC. This contribution is significant because it will (i) define the molecular mechanism by which Plk1 regulates the mTOR/AR signaling; (ii) genetically evaluate how Plk1 cooperates with loss of PTEN signaling; and (iii) validate Plk1 as a critical therapeutic target to enhance the efficacy of metformin. The research is innovative as it approaches the disease from a novel Plk1 signaling pathway, challenging the traditional view that Plk1 functions solely to regulate mitotic events. These studies are poised to provide a new paradigm for improved patient therapies by identifying the key regulator of the mTOR/AR signaling that is critical for generating and maintaining the CRPC phenotype.